Mochikomi refers to stranded stones--stones which end up within the opponent's territory with virtually no aji.
Some Western players have adopted mochikomi to refer to a failed invasion, perhaps by analogy to the term uchikomi. But the original Japanese sense of mochikomi, as mentioned above, is stones--even just one--which, by virtue of future moves by either side, end up floating, aji-less, within enemy territory.
In Japanese, mochikomi ni naru (my stone(s) ended up stranded), or mochikomi ni sareru (my stone(s) were stranded).